1. Field of the Invention
This invention relates to the field of transport photonics. More specifically the invention relates to physical layer transport testing of the integrity of optical network components.
2. Background Art
Originally, communication networks were constructed of copper wires for transmission of electrical signals to communicate data and sound. To facilitate communication, Ethernet technology was developed to aid in interconnecting electronic equipments. The Institute of Electrical and Electronics Engineering (IEEE) subsequently adopted the Ethernet and provided basically an open system standard such that products developed by different vendors, in compliance with the standard, are able to communicate with each other and transfer data from point-to-point over twisted pair cables (e.g., Category 5 cable). Thus today a majority of networks use Ethernet standards for communication over copper wires. Standards currently in use for Local Area Networks (LAN) and some Metro Area Networks (MAN) include the 10 Megabit, 100 Megabit, and 1 Gigabit Ethernet.
However, the demand created by a larger number of customers, need for increased data transfer rates, as well as increased communication distances, resulted in the proliferation of fiber optic transmission networks. Fiber optic transmission was attractive because of the very limited data carrying capability (i.e., low bandwidth) of electrical signal transmission medium (e.g., copper) in contrast with an optical transmission medium. Moreover, electrical based communication systems suffer from power losses (due to diffusion and skin effect) that accompany copper based transmission lines. Thus, in the telecommunications technology, optical fibers and optical fiber cables have become the transmission media of choice, primarily because of the tremendous bandwidth capabilities and low power loss associated with optical fibers. To this effect, faster and faster communication standards are being developed to feed this insatiable demand for speed. One of these standards is the 10-Gigabit Ethernet standard.
The 10 Gigabit Ethernet Standard extends the current IEEE 802.3 protocols to an operating speed of 10 Gigabits per second. The new protocol also allows for expansion of Ethernet applications to include Wide Area Networks (WAN). Unlike earlier Ethernet Standards, the 10-Gigabit Ethernet Standard, known as the IEEE 802.3ae, will only function over fiber optic transmission lines and only operate in full duplex mode therefore rendering collision detection protocols unnecessary.
A consortium of companies formed an alliance, the 10 Gigabit Ethernet Alliance, to promote this new standard for optical fiber transmission. The descriptions that follow are freely adopted from white papers published by the 10 Gigabit Ethernet Alliance.
The 10-Gigabit Ethernet (10 GbE) technology represents the merging of technologies between telecommunications and data communications. The IEEE 802.3ae Standard is a 10-Gigabit Ethernet standard developed to ensure interoperability between products from different vendors using optical fiber transmission medium for data communication. The 10 GbE does not obsolete current network infrastructure because it is designed to be interoperable with other networking technologies such as SONET (Synchronous Optical Network). Thus the standard enables Ethernet packets to travel across SONET links with very little inefficiency.
SONET is the American National Standards Institute standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). Together, they ensure standards so that digital networks can interconnect internationally and that existing conventional transmission systems can take advantage of optical media through tributary attachments.
The 10-Gigabit standard uses the Ethernet's Media Access Control protocol, the frame format, and the minimum and maximum frame size of the IEEE 802.3. The standard is a full duplex and fiber only technology therefore it does not require carrier-sensing, multiple-access with collision detection (CSMA/CD) protocol of slower, half-duplex Ethernet technologies.
Under the International Standards Organization's Open Systems Interconnection (OSI) model, Ethernet is basically a 2-layer protocol: the physical layer (PHY), and the Media Access Control Layer (MAC). The 10-Gigabit Ethernet Standard adopts cost effective and robust technologies to minimize transitional costs to the 10-Gigabit Ethernet. For instance, the standard basically specifies the physical layer while specifying only minimal changes to current Media Access Control devices.
An Ethernet PHYsical layer device (PHY), which corresponds to layer 1 of the OSI model, connects through an optical or copper medium to the MAC layer, which corresponds to OSI layer 2. The OSI Ethernet model is illustrated in FIG. 1. As illustrated, block 100 represents the physical layer device and block 102 represents the MAC layer device. The Ethernet structure further divides the PHY, block 100, into a Physical Media Dependent sublayer (PMD) 104, Physical Media Attachment (PMA) 110, and a Physical Coding Sublayer (PCS) 106. Optical transceivers, for example, are PMDs. Thus the PMD converts optical signals to electric and vise versa. PMA 110 is a serializer/deserializer sublayer. And finally, PCS 106 comprises coding (e.g., 64B/66B or 8B/10B) and serializer or multiplexing functions.
Between the MAC and the PHY is XGMII 108, or 10 Gigabit Media Independent Interface (XGMII), 108. The XGMII provides simple, inexpensive, and easy to implement interconnection between the MAC sublayer and the PHY. Transmission lines 101 and 103 carry full duplex XGMII traffic at 10 Gb/second between the MAC and the PHY. Each direction is independent and contains a 32-bit data path, as well as clock and control signals. In total, the XGMII is a 74-bit wide bus. A 10 Gigabit Attachment Unit Interface (XAUI) is optionally used in place of or to extend the operational distance of the XGMII with reduced pin count (see Clause 47 of the IEEE 802.3ae Standard).
XAUI is a full duplex interface that uses four self-clocked serial differential links in each direction to achieve 10 Gb/s data throughput. Each serial link operates at 3.125 Gb/s to accommodate both data and overhead associated with 8B/10B coding. The XAUI interface is self-clocked thus eliminating any skew concerns between the clock and data, and extends the functional reach of the XGMII by approximately another 50 cm (from 7 cm). Conversion between the XGMII and XAUI occurs at a sublayer called the XGMII Extender Sublayer (XGXS).
In keeping with the Ethernet standard of hot-pluggable transceivers, a Multi-Source Agreement consortium called XENPAK was formed to develop a standard requirement for hot-pluggable transceivers. FIG. 2 is an illustration of the building blocks of a standard XENPAK 10 Gigabit Ethernet transceiver. The Ethernet transceiver, referred to as the XENPAK module, is represented by block 216. XENPAK module 216 comprises the PMD sublayer 212, the PMA (Physical Medium Attachment) Sublayer 210, PCS 208, and XGXS 206.
As illustrated in FIG. 2, the optical medium is connected through Media Dependent Interface (MDI) 211 to the PMD (i.e., 212). The PMD converts the optical signal to serial electrical data. The PMA sublayer, 210, is a serializer/deserializer sublayer which converts the serial data to a 16-bit parallel signal at 209 for connection to PCS 208. PMA 210 also converts the 16 bit signal from the PCS to 1-bit serial for the PMD. PCS 208 does the encoding function such as 8B/10B or 64B/66B and provides the coded XGMII output at 207. XGXS 206 converts the 32-bit wide XGMII receive data to differential XAUI interface data at 205. It also converts the XAUI data 205 to XGMII. At the XAUI interface 205 is a XENPAK male connector from where signals in the XAUI lanes may be tapped. For instance, the MAC layer in the controller may have a XENPAK female connector so that the transceiver may easily plug into the controller device. The XAUI interface is preferred because it eliminates the need for 36+36 bit wide Tx and Rx Printed Circuit Board (PCB) traces which could result in skew problems thereby increasing the complexity of tests required to certify each 10 Gbe equipment. XAUI has a set of tests specified by the consortium for testing the integrity of the XAUI signals.
The MAC layer has its own XGXS sublayer (i.e., 204) to convert the XAUI lane signals back to XGMII since MAC Engine 202 interface is XGMII signals. Thus, the MAC engine may be used to test the integrity of the XAUI signals for example. A wide range of required tests are described in the IEEE 802.3ae Standard for 10 Gbe equipments. These tests include optical transceiver and receiver testing, electrical XAUI tests, and jitter tests.
Electrical XAUI tests require interface and ability to receive and transmit signals from the XAUI lanes. Any testing entails some form of destruction of data. Thus tapping into the XAUI lanes may compromise the integrity of the data. Therefore, good test equipments try to minimize the destructive impact on the actual data. FIG. 3 is an illustration of prior art equipments for testing integrity of XAUI lane traffic.
The test configuration includes a Device Under Test (DUT) 314, a XAUI Traffic Generator and Analyzer 302, and an SMA Adapter 308. DUT 314 includes connector 312 which provides connection to the electrical XAUI signals. Connector 312 comprises sixteen SMA connections: four differential pairs for transmit (eight total lines), and four differential pairs for receive (eight total lines). Eight copper cables 311 and eight copper cables 313 are connected to the SMA connectors of connector 312 to provide access to the XAUI signals. DUT 314 also contains XENPAK module 216 which provides access to optical transmission through its Tx and Rx terminals.
Prior art test equipments are specialized equipments that have connectors to receive the electrical signal. For instance, prior art XAUI Traffic Generator and Analyzer 302 includes connector 304 for receiving the electrical signals originating from the XAUI lanes. One problem is that prior art test equipments may not have the compatibility to directly receive XAUI electrical signals thus an adapter is typically used. In the illustration of FIG. 3, SMA adapter 308 (sometimes called break-out boxes) is used to convert the XAUI electrical signals from the SMA connectors to the test equipment connectors. For instance, the SMA Adapter 308 receives the differential XAUI electrical signals at connector 310. Connector 310 comprises eight pairs of SMA connectors, each pair carrying a differential XAUI electrical signal. The differential XAUI signals is then converted in SMA Adapter 308 and provided at connector 306. Connector 306 is compatible with connector 304 on test equipment 302. Transmission line 305, which typically comprises of copper wires, carries the electrical signal between the test equipment (i.e., 302) and the SMA Adapter (i.e., 308). Transmission line 305 and connector 306 combine to cause additional reflection losses and degradation of the XAUI electrical signals.
Transmission line 305 comprises a group of copper wires (cables) used to convey electrical data signals between test equipment 302 and the break-out box 308. Thus, there are additional elements, cable and connector, between the test equipment and the DUT. These additional elements contribute to destruction of the XAUI lane signals.
A cable is supposed to accurately convey a signal however losses accumulate along the cable path as the signal travels from one point to another because of imperfections. These imperfections are not necessarily due to manufacturing problems, but are mostly due to the physical nature of a cable. Like other physical devices, cables exhibit some losses when a signal is transmitted through them. Thus, the longer the cable length, the more losses accumulate. The accumulated transmission loss is known as “insertion loss” by those of ordinary skill in the arts. Additionally, connector 306, which is in the transmission path, contributes to the accumulated signal loss and thus more destruction of the XAUI lane signals.
Thus, a method for reliably and cheaply testing the integrity of 10 Gigabit Ethernet transmission components is desired.